The invention relates to electroerosion printing and to recording materials exhibiting improved lubricity for use in such processes, and especially for use in either direct-negative applications or the production of direct offset masters.
Electroerosion printing is a well-known technique for producing markings, such as, letters, numbers, symbols, patterns such as circuit patterns, or other legible or coded indicia on recording material in response to an electric signal which removes or erodes material from the surface of the recording material as the result of spark initiation.
The surface which is eroded or removed to provide such indicia on the recording material is usually a thin film of conductive material which is vaporized in response to localized heating associated with sparking (arcing) initiated by applying an electric current to an electrode in contact with the surface of a recording material comprising the thin conductive film on a nonconductive backing or support. In the present state of the technology the thin conductive film is usually a thin film of vaporizable metal, such as aluminum.
Electroerosion printing is effected by the movement of a stylus or a plurality of styli relative to the surface of specially prepared recording media. Electrical writing signals are fed to the stylus to provide controlled electrical pulses which generate sparks at the surface of the recording material to selectively heat and remove by evaporation a layer of the recording materal. The locations from which material is removed correspond to the indicia or images which are to be recorded.
In the course of this process, the stylus is moved relative to a surface of the recording material and in contact with the removable layer, i.e., the thin film of vaporizable material.
In high speed and high resolution printing processes there may be thirty or more styli arranged to form a print head. As the styli are actuated by computer controlled voltage pulses, arcing and consequently high local temperatures cause evaporation of the conductive layer to record the desired information.
Due to the fragility of the thin conductive layer and limitations in the precise adjustment of styli pressure, in a multi-styli head considerable scratching (undesired removal of the removable layer) is observed to take place during high resolution electroerosion printing.
It has been recognized for some time, therefore, that the use of a lubricant and/or protective overcoat on the surface of such electroerosion recording material would be helpful to reduce scratching by the styli. Application of lubricants comprising long chain fatty acids such as stearic acid, palmitic acid etc., were found to reduce the scratching problem somewhat but considerable stylus scratching of the thin aluminum film of electroerosion recording materials continues to be observed. Therefore, efforts have been directed to finding a superior lubricant-protective layer composition for the surface of electroerosion recording materials.
In co-pending application Ser. No. 454,744 filed Dec. 30, 1982, and filed in the name of Mitchell S. Cohen, one of the coinventors herein, there are described graphite-cellulosic binder based overlayer compositions for electroerosion recording materials. Among the various laminar solids and other soft compounds and soft metal particles disclosed in Ser. No. 454,744 for use as the lubricant conductive particles graphite, due to its cost, effectiveness and easy dispersibility, is the preferred material described therein.
While the lubricant overlayer compositions described previously provide substantial improvement in print quality, it is desirable that still better systems be developed so as to eliminate the "baking" and "fouling" problem caused by accumulation of the eroded organic/inorganic debris on the print-head.
It has been found desirable to reduce the conductivity of the graphite-binder overcoat in order to achieve better print definition. In addition, it has been desired to provide overcoats for electroerosion recording materials characterized by low surface energy, improved mechanical strength, and good lubricity, and improvements in terms of reduced stylus fouling and reduced head wear. Also and that debris generated during the printing process should not be adherent to the print-head.
One approach considered for lowering the conductivity of the graphite-polymeric binder protective overcoat was to increase the binder content of the protective layer. However, with this approach, there is a problem of stylus fouling caused by accumulation of organic polymeric/oligomeric debris generated during the electroerosion process.
Among prior disclosures relevant to electroerosion printing, U.S. Pat. No. 2,983,220, Dalton et al, discloses a lithographic coating on an electroerosion recording sheet; the coating may be a copolymer containing zinc oxide and zinc sulfide. An internal layer containing conductive material, such as, graphite is disclosed in U.S. Pat. No. 3,048,515, Dalton. An electroresponsive recording blank having a removable masking layer containing a luminescent material is described in U.S. Pat. No. 2,554,017, Dalton. Other prior art providing further general background in the field of electroerosion printing includes U.S. Pat. Nos. 3,138,547, Clark and 3,411,948, Reis. High temperature lubricants comprising graphite in oil are also known, as is described in U.S. Pat. No. 3,242,075, Hunter.